The present invention relates to current cells for providing precisely controlled currents, and more particularly to a current cell and method for providing a specified current for a digital-to-analog converter.
The need to provide a precise amount of current is found in many types of electronic devices, including a digital-to-analog converter (DAC) in which an incorrect translation of a digital input bit to a current can cause an inaccurate output. A DAC, for example, may include over 20 current sources with transistors and resistors. A mismatch among these components can cause an undesirable nonlinear output from the DAC.
Electronic devices which require precise currents include components such as the transistors and resistors which are manufactured in processes which may not be sufficiently accurate to provide the precise current required. To this end, the art has developed so that corrective steps are included in the manufacturing process of such devices which obviate the inaccuracies of the processes. The corrective steps include changing the resistance of the resistors which are influential in determining current accuracy.
A resistor is typically manufactured with a resistance that is not sufficiently precise for electronic devices, such as DACs, which require precise currents. However, during the device manufacturing process the resistance of the resistor can be changed by trimming so that it does provide sufficiently accurate current. Trimming typically involves the physical removal of a portion of the resistor, such as with a laser, so that the resistor's resistance increases. Physical trimming requires that the device be tested and trimmed before it is enclosed, and the testing, trimming and enclosure steps can introduce further inaccuracies. Further, resistance cannot be decreased through physical trimming and thus every current cell in a DAC must be trimmed so that the output current matches the output current from the lowest untrimmed cell.
Trimming may also be electronic in that current may be added or removed through extra circuitry which is added to the electronic device specifically to do the trimming. Electronic trimming may be carried out after the device is enclosed and may increase or decrease current, but the trimming circuitry takes up valuable space and uses leads which are not otherwise available for substantive functions.
Accordingly, it is an object of the present invention to provide a novel current cell and method in which the need to trim resistors is reduced and in which physical trimming may either reduce or increase output current so as to obviate the problems of the prior art.
It is another object of the present invention to provide a novel cell and method for providing an output current at a desired level by physically trimming one or the other of two series connected trimmable resistors to increase or decrease the output current to the desired level.
It is yet another object of the present invention to provide a novel cell and method for providing a desired current level in which a first circuit reduces deviation of the output current from the desired level, the first circuit having a pair of series-connected fixed resistors, and in which a second circuit has a series-connected pair of trimmable resistors, one or the other of which may be trimmed to adjust the output current from the first circuit to the desired level.
It is still another object of the present invention to provide a novel method and cell for providing a desired current level in which the cell has a first circuit with plural transistors having a control terminal at a first potential and collectors connected to an output current node, each of the first transistors having a second terminal connected to an end of a different one of plural series-connected pairs of untrimmed resistors, wherein each of the pairs of untrimmed resistors is connected to every other of the pairs at a common node between the untrimmed resistors, and in which a second circuit for trimming the output current from the first circuit to the desired level includes a transistor having a first terminal connected to an end of a series-connected pair of trimmable resistors, in which a common node of the pair of trimmable resistors is connected to the common node of the pairs of untrimmed resistors.
It is yet a further object of the present invention to provide a novel method and cell for providing a desired current level in which the cell provides an output current from a current switch at the desired level, and in which a cascode transistor establishes a common V.sub.CE for the transistors in the cell.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.